half of the reason to circulate durring the main mash is for the vourlauff (sp?) effect of clearing the particulate out of the wort. if you pumped in reverse, all the particles would be deposited at the bottom of the grain bed. then when you went to sparge, they would all come out into the boil kettle where you dont want them.
i dont think reverse sparging would work very well. you would probably have to reverse the flow back to top-to-bottom in order to sparge.
ive pumped into the bottom before to clear a stuck sparge. it does work for that, its just not something to do as a normal practice. if you are getting stuck sparges (mashes?), you should work on your false bottom, or adjust your flow rate to avoid grainbed compaction instead.
